Needs Render

I am trying to figure out what happened here with my workflow... I was happily editing away this man talking to camera. One of the wider shots required him being moved off to the left (As you can observe in the Viewer (In the attached screenshot) with a white background. At first FCP Didn't seem to mind doing playback on the white background composite on the fly and then for some reason it started refusing to playback without rendering first.

Is there some setting I changed by accident? What can I do to fix this?

Doh! Totally forgot about that. Thanks, Dave. In answer to your question about the pure white our DP was told to essentially create an apple commercial style blown out style white void space--a popular aesthetic. It was not created in post.